Biomedical subjects

A J van Soest

Publications and source records attributed to A J van Soest.

16 recordsLinked to original sources

PubMed2005

Factors underlying the perturbation resistance of the trunk in the first part of a lifting movement.

In the first part of lifting movements, the trunk movement is surprisingly resistant to perturbations. This study examined which factors contribute to this perturbation resistance of the trunk during lifting. Three possible mechanisms were studied: force-length-velocity characteristics of muscles, the momentum of the trunk as well as the effect of passive extending of the elbows. A forward dynamics modelling and simulation approach was adopted with two different input signals: (1) stimulation of Hill-type muscles versus (2) net joint moments. Experimental data collected during an unperturbed lifting movement were used as a reference, which a simulated lifting movement had to resemble. Subsequently, the simulated lifting movement was perturbed by applying 10 kg extra mass at the wrist (both before and after lift-off and with/without a fixed elbow), without modifying the input signals. The momentum of the trunk appeared to be insufficient to explain the perturbation resistance of trunk movements as found experimentally. In addition to the momentum of the trunk, the force-length-velocity characteristics of the muscles are necessary to account for the observed perturbation resistance. Initial extension of the elbow due to the mass perturbation delayed the propagation of the load to the shoulder. However, this delay is reduced due to the impedance at the elbow provided by the characteristics of muscles spanning the elbow. So, the force-length-velocity characteristics of the muscles spanning the elbow joint increase the perturbation at the trunk.

Biomechanical Phenomena↗

PubMed2005

Consequences of ankle joint fixation on FES cycling power output: a simulation study.

INTRODUCTION: During fixed-ankle FES cycling in paraplegics, in which the leg position is completely determined by the crank angle, mechanical power output is low. This low power output limits the cardiovascular load that could be realized during FES ergometer cycling, and limits possibilities for FES cycling as a means of locomotion. Stimulation of ankle musculature in a released-ankle setup might increase power output. However, releasing the ankle joint introduces a degree of freedom in the leg that has to be controlled, which imposes constraints on the stimulation pattern. METHODS: In this study, a forward dynamics modeling/simulation approach was used to assess the potential effect of releasing the ankle on the maximal mechanical power output. RESULTS: For the released-ankle setup, the optimal stimulation pattern was found to be less tightly related to muscle shortening/lengthening than for the fixed-ankle setup, which indicates the importance of the constraints introduced by releasing the ankle. As a result, the maximal power output for 45-RPM cycling in the released-ankle setup was found to be about 10% lower than with a fixed ankle, despite the additional muscle mass available for stimulation. Power output for the released-ankle setup can be improved by tuning the point of contact between the foot and pedal to the relative strength of the ankle plantar flexors. For the model used, power output was 14% higher than for the fixed-ankle setup when this point of contact was moved posteriorly by 0.075 m. CONCLUSION: Releasing the ankle joint and stimulating the triceps surae and tibialis anterior is expected to result in a modest increase in power output at best.

Acceleration↗

PubMed2003

Stability of bipedal stance: the contribution of cocontraction and spindle feedback.

The aim of this study is to assess the contribution of cocontraction and spindle feedback to local stability during bipedal stance. To that aim, an existing nonlinear state space model of the human musculoskeletal system is linearized in a reference equilibrium state. The maximal real part of the eigenvalues of the linearized system matrix A and the low-frequency joint stiffness are used as a measure of local stability. Muscle properties, as represented in a Hill-type muscle model, are shown to improve the behavior, the improvement being larger at high cocontraction. However, even at maximal cocontraction the low-frequency joint stiffness generated by the muscle properties is insufficient to yield a locally stable system. It follows that feedback is necessary to ensure local stability. In this study, the potential contribution of spindle feedback is investigated by optimizing the feedback gains for contractile element length and velocity for each muscle. It is found that in the case of time-delayed negative feedback, it is impossible to stabilize the system on the basis of spindle feedback. When positive time-delayed feedback is allowed, a barely stable system is obtained. When the time delays are removed, the feedback gains can be chosen such that a locally stable system is obtained, indicating the limitations imposed by the presence of time delays. Finally, it is shown that for small perturbations the response of the linear system to an arbitrary perturbation is similar to that of the nonlinear system, indicating the validity of the approach used. It is concluded that the combination of muscle properties and time-delayed spindle feedback is insufficient to obtain a system with reasonable local stability.

Feedback↗

PubMed2002

A computer-graphics model of muscle activation and contraction dynamics.

An interactive computer-graphics model of the mechanical interaction between the structural components of a Hill-type muscle model is presented. The model allows the length, velocity, and force in the contractile component, parallel elastic component, and series elastic of a generic (normalised) muscle-tendon complex to be computed from input defining instantaneous muscle stimulation and muscle-tendon length. By altering model inputs and model parameter values the user can observe corresponding model behaviour in schematic and graphical form. In this paper a general description of the model is provided together with examples of how the model can be used to simulate different contractile conditions. It is intended that the model will serve as a useful tool for demonstrating the behaviour of Hill-type muscle models.

Computer Graphics↗

PubMed2001

Why do people jump the way they do?

When humans perform maximum height squat jumps, their segmental rotations contribute in a proximodistal sequence to the vertical acceleration of the center of gravity. The same kinematic pattern occurs in a forward dynamic model of the musculoskeletal system when muscle stimulation is optimized to maximize jump height. This paper examines why this kinematic pattern maximizes jump height in humans, given the design of the human musculoskeletal system.

Biomechanical Phenomena↗

PubMed2000

Spontaneous kicking behavior in infants: age-related effects of unilateral weighting.

The age-related effects of unilateral weighting on spontaneously generated kicks in 18 healthy, full-term infants were investigated. The main question was whether infants during the first half-year after birth reveal changes in how they adjust to unilateral weighting. At 6 weeks, infants reduced the frequency of the weighted leg and increased that of the unweighted leg whereas at 12 weeks the frequency of kicking increased in both legs. At both ages, unilateral weighting also resulted in differences on a number of kinematic parameters. By 18 and 26 weeks, such frequency and kinematic effects were no longer present. With regard to interlimb couplings, a clear pattern of bilateral coordination was only present at 26 weeks; these results suggest that the effects of unilateral weighting are not directly related to the tightness of interlimb couplings. The implications of these age-related differences for understanding developmental changes in the control of leg movements are discussed. It is suggested that the infants' improved ability to act in a task-specific manner as well as nonlinear changes in the musculo-skeletal system and fine-tuning processes at a neural level might be factors of importance.

Age Factors↗

PubMed2000

Development of spontaneous leg movements in infants with and without periventricular leukomalacia.

The main question asked in the present study was whether support could be found for the notion that supraspinal influences on the generation of spontaneous kicking movements become increasingly apparent in the first half-year after birth. In comparing groups of infants with and without damage in tracts connected with the cortex surrounding the central sulcus, such support would consist of the finding that similar patterns of spontaneous kicking are observed early in development, whereas differences between groups should occur with increasing age. Using 3-D registrations, the spontaneous kicking movements of 19 infants with differing degrees of periventricular, lobar, and subcortical leukomalacia based on white matter (WM) abnormalities on ultrasound were compared to those of 10 healthy control infants at 6, 12, 18, and 26 weeks of corrected age. Magnetic resonance imaging recordings were used to identify the location and severity of the brain lesions. Infants with extensive lesions in the periventricular and lobar WM with or without diffuse lesions in the subcortical WM showed a decreased variability on some spatial and temporal parameters of kicks. More importantly, these infants showed a different developmental course for intralimb couplings when compared to the other infants; they were unable to dissociate tight intralimb couplings at 18 and 26 weeks. As all of these infants had substantial damage of the corticospinal tracts, these findings suggest that these tracts are involved in the regulation of intralimb joint dissociations between 4 and 6 months of age. However, caution is needed as areas outside those in which the corticospinal tracts are located could be damaged as well and most of the infants with moderate to severe lesions in the corticospinal tract had additional psychomotor problems. For interlimb couplings and most of the spatial and temporal parameters of kicks, no differences were found between groups. This strengthens the claim that inter- and intralimb couplings are organized in fundamentally different ways.

Analysis of Variance↗

PubMed2000

Which factors determine the optimal pedaling rate in sprint cycling?

INTRODUCTION: Mechanical power output in sprint cycling depends on pedaling rate, with an optimum at around 130 revolutions per minute (rpm). In this study, the question is addressed if this optimal pedaling rate can be understood from a Hill-type description of muscular dynamics. In particular, it is investigated how 1) the power-velocity relationship that follows from Hill's force-velocity relationship and 2) activation dynamics (from the perspective of which the optimal pedaling rate is near-zero) affect the optimal pedaling rate. METHODS: A forward dynamics modeling/simulation approach is adopted in this study. The skeletal model is a 2D linkage of rigid segments; it is actuated by eight Hill-type "muscles." Input of the model is the neural stimulation of the muscles, output is the resulting movement and variables dependent thereupon, such as pedal forces. For a wide range of isokinetic pedaling rates, the neural stimulation is optimized with respect to the average mechanical power output. RESULTS: Correspondence between experimental data and simulation results regarding 1) the (pedaling-rate dependent) muscle phasing, 2) pedal forces, and 3) the power-pedaling rate relationship is good. At the optimal pedaling rate predicted by the model (120 rpm), muscles contract at velocities well below those that maximize their power output. Finally, when a model is considered that lacks activation dynamics, it is found that both the optimal pedaling rate and the maximal power output increase substantially. DISCUSSION: From the results pertaining to the standard model, it is concluded that the optimal pedaling rate is not uniquely specified by the power-velocity relationship of muscle, as suggested in literature. From the results pertaining to the model lacking activation dynamics, it follows that activation dynamics plays a surprisingly large role in determining the optimal pedaling rate. It is concluded that the pedaling rate that maximizes mechanical power output in sprint cycling follows from the interaction between activation dynamics and Hill's power-velocity relationship.

Bicycling↗

PubMed2000

Two-joint muscles offer the solution, but what was the problem?

Prilutsky's paper is mainly concerned with the coordination of one- and two-joint muscles. This commentary on the paper addresses the question why we have two-joint muscles in the first place. From an evolutionary point of view, two-joint muscles must have contributed to fitness by presenting a solution to problems that could not be solved with musculoskeletal systems comprising only one-joint muscles. One such problem, not mentioned by Prilutsky, is the following. In a system equipped with only one-joint muscles, satisfying directional constraints would demand, in certain phases of movements, deactivation of muscles that are shortening. Consequently, the work output of these muscles would be limited. The incorporation of two-joint muscles helps to overcome this problem. The reason is that it offers the possibility to redistribute energy across joints, thereby making it possible to accomplish more successfully the difficult task of producing work while steering the movement.

Humans↗

PubMed1996

Influence of the parameters of a human triceps surae muscle model on the isometric torque-angle relationship.

This study investigates the influence of parameter values of the human triceps surae muscle on the torque-angle relationship. The model used consisted of three units, each containing a contractile, a series elastic and a parallel elastic element. Parameter values were based on morphological characteristics, which made it possible to model individual units. However, for a number of parameters the values reported in the literature vary considerably. It was investigated how sensitive model results were for variation of these parameters. Slack length of the series elastic element, mean moment arm, maximum force, and length of the contractile element appeared to be the most important determinants of the behavior. For mean moment arm and contractile element length, morphology-based methods of estimation could be recommended. Slack length and maximum force were obtained through optimization. It was concluded that the model does not contain parameters on which its output depends strongly and which are difficult to estimate as well, with two exceptions: slack length of the series elastic element and maximum force.

Ankle↗

PubMed1995

The effect of tendon on muscle force in dynamic isometric contractions: a simulation study.

Recently, Baratta and Solomonow J. Biomechanics 24, 109-116 (1991) studied the effect of tendon on muscle-tendon complex behavior in cat tibialis anterior (TA) muscle. This was done by determining the relation between neural stimulation and muscle force in a dynamic isometric experiment, both before and after the removal of the distal tendon. From their results, Baratta and Solomonow concluded that in isometric and concentric contractions at mid-range force levels, tendon behaves as a rigid force conductor. This conclusion is in conflict with literature in which several functions are attributed to the elastic behavior of the series elastic element (SEE), of which tendon is the major part. The present study investigates the expected generalizability of their findings, by simulating the experiments using a straightforward Hill-type muscle model. First, model predictions are shown to be in line with the experimental results on cat TA: in dynamic isometric experiments at mid-range force levels, the effect of SEE removal is indeed negligible. Second, the effect of SEE removal is predicted to vary largely among muscles. Third, the most important determinants of the effect of SEE removal in dynamic isometric contractions are shown to be maximum fiber shortening velocity and the ratio of SEE slack length to fibre optimum length.

Animals↗

PubMed1993

The contribution of muscle properties in the control of explosive movements.

Explosive movements such as throwing, kicking, and jumping are characterized by high velocity and short movement time. Due to the fact that latencies of neural feedback loops are long in comparison to movement times, correction of deviations cannot be achieved on the basis of neural feedback. In other words, the control signals must be largely preprogrammed. Furthermore, in many explosive movements the skeletal system is mechanically analogous to an inverted pendulum; in such a system, disturbances tend to be amplified as time proceeds. It is difficult to understand how an inverted-pendulum-like system can be controlled on the basis of some form of open loop control (albeit during a finite period of time only). To investigate if actuator properties, specifically the force-length-velocity relationship of muscle, reduce the control problem associated with explosive movement tasks such as human vertical jumping, a direct dynamics modeling and simulation approach was adopted. In order to identify the role of muscle properties, two types of open loop control signals were applied: STIM(t), representing the stimulation of muscles, and MOM(t), representing net joint movements. In case of STIM control, muscle properties influence the joint moments exerted on the skeleton; in case of MOM control, these moments are directly prescribed. By applying perturbations and comparing the deviations from a reference movement for both types of control, the reduction of the effect of disturbances due to muscle properties was calculated. It was found that the system is very sensitive to perturbations in case of MOM control; the sensitivity to perturbations is markedly less in case of STIM control. It was concluded that muscle properties constitute a peripheral feedback system that has the advantage of zero time delay. This feedback system reduces the effect of perturbations during human vertical jumping to such a degree that when perturbations are not too large, the task may be performed successfully without any adaptation of the muscle stimulation pattern.

Computer Simulation↗

PubMed1993

The influence of the biarticularity of the gastrocnemius muscle on vertical-jumping achievement.

Hypotheses concerning the influence of changes in the design of the human musculoskeletal system on performance cannot be tested experimentally. Computer modelling and simulation provide a research methodology that does allow manipulation of the system's design. In the present study this methodology was used to test a recently formulated hypothesis concerning the role of the biarticularity of the gastrocnemius muscle (GAS) in vertical jumping [Bobbert and van Ingen Schenau, J. Biomechanics 21, 249-262 (1988)]. This was done by comparing maximal jump heights for a model equipped with biarticular GAS with a model equipped with a monoarticular GAS. It was found that jump height decreased by 10 mm when GAS was changed into a monoarticular muscle. Thus, the hypothesis formulated by Bobbert was substantiated, although quantitatively the effect is small. Our result differs from that of Pandy and Zajac [J. Biomechanics 24, 1-10 (1991)], who performed similar model calculations. It is shown that the results described by these authors can be explained from the moment-arm-joint-angle relation of GAS at the knee in their model.

Ankle Joint↗

PubMed1992

SPACAR: a software subroutine package for simulation of the behavior of biomechanical systems.

Direct dynamics computer simulation is gaining importance as a research tool in the biomechanical study of complex human movements. Therefore, the need for general-purpose software packages with which the equations of motion can be derived automatically and solved numerically is growing. In this paper such a method is described: SPACAR. The method is compared to well-known commercially available software packages. On the basis of the results obtained on a test problem simulated with both SPACAR and DADS, it is concluded that both methods are accurate; DADS is much faster. The user-friendliness of SPACAR is less than that of DADS. However, SPACAR has two major advantages. First is the basic deformability of all elements, which allows handling of all kinds of problems within a unified framework; second is the full availability of the source code, which allows the experienced user to broaden the scope of possibilities to any extent.

Biomechanical Phenomena↗

PubMed1990

The role of series elastic structures in prestretch-induced work enhancement during isotonic and isokinetic contractions.

The influence of series elastic structures on the dynamics of the contractile machinery was examined in the gastrocnemius medialis (GM) of five male Wistar rats, with respect to enhancement of work of a muscle-tendon complex after active stretch. Imposed isotonic and isokinetic contractions were preceded by either an isometric phase (PI) or an active stretch (PS). The effects of fibre length differences at the onset of shortening, due to differences of extension of tendinous structures, were studied. For the isotonic experiments fibre length and shortening velocity were estimated 30 ms after release and compared with the PI length-velocity curve determined at the same force level. For shortening above the optimum length, about half of the enhanced shortening found after prestretch could be explained by PS-PI fibre length differences. Below the optimum length, PS shortening velocity was somewhat lower than expected on the basis of length-velocity characteristics. Enhancement of work output due to stretch was different for isokinetic and isotonic shortening. In isokinetic shortening, following prestretch, fibre work was limited because of enhanced shortening of the tendinous structures. In stretch-shortening cycles imposed on a muscle-tendon complex, the length of the complex affected all prestretch effects, i.e. potentiation of the contractile element, contractile element interaction with the tendinous structures, and elastic energy release. It is concluded that, besides potentiation effects and enhanced elastic energy release, the influence of series elastic structures on fibre dynamics determines active stretch-induced work enhancement. The contribution by these mechanisms to this work enhancement depends largely on the type of stretch-shortening cycle.

Animals↗

PubMed1985

A comparison of one-legged and two-legged countermovement jumps.

Ten well-trained male volleyball players performed one-legged and two-legged vertical countermovement jumps. Ground reaction forces, cinematographic data, and electromyographic data were recorded. Jumping height in one-legged jumps was 58.5% of that reached in two-legged jumps. Mean net torques in hip and ankle joints were higher in one-legged jumps. Net power output in the ankle joint was extremely high in one-legged jumps. This high power output was explained by a higher level of activation in both heads of m. gastrocnemius in the one-legged jump. A higher level of activation was also found in m. vastus medialis. These differences between unilateral and bilateral performance of the complex movement jumping were shown to be in agreement with differences reported in literature based on isometric and isokinetic experiments.

Ankle Joint↗